1. Field of the Invention
The present invention concerns an asynchronous switching network for a communication installation including packet mode units and more particularly a traffic control system for a network of this kind.
2. Description of the Prior Art
French patent application FR-A-2 665 314 (corresponding to European Patent No. 0468442) describes an integrated services digital network communication installation or interconnect network by which the dedicated modular units constituting the core are connected by point-to-point links according to their communication requirements.
A network of this kind can incorporate an asynchronous subnetwork of packet mode units, i.e. the units within the core directly or indirectly involved in switching data packets.
These data packets are conventionally switched by centralized switches or by a distributed network formed by a bus or by a loop, each switch including a memory in which incoming packets are temporarily stored until they are forwarded to destinations depending on address information included in their respective header.
The switches include a plurality of inputs at which they can receive packets and the latter are switched to outputs corresponding to their respective destinations. Each output can therefore transmit a succession of packets which may reach it from different inputs, the order in which packets are transmitted from the same output being set by predetermined criteria including the arrival time and priority of each packet.
Packets awaiting transmission from an output are therefore temporarily stored in memory in the switch until they are transmitted. To the extent that the packet senders are independent of each other, the number of packets arriving at the various inputs and to be passed through an output of the switch can exceed the handling capacity of their output if no specific precautions are taken to prevent this. There is then the risk of packets being lost in the switches and losses can occur if a large number of asynchronous terminals attempt simultaneously to communicate with the same asynchronous server via the same port, for example.
Provisions are therefore made to reduce the risk of packet loss at the switches, there being a particularly high risk of packet loss from the memories in which the packets are temporarily stored. These memories are conventionally of the stack type and have a capacity limited for reasons of size and cost. A first way to limit the risk of packet loss is conventionally employed in large asynchronous networks and consists in having users served by switches in parallel so as to reduce the risk by spreading it. This first approach is generally associated with measures for limiting input traffic allowing for differences between predetermined traffic values and values reflecting the actual traffic. This entails the use of a set of means which it is usually not feasible to incorporate in private type installations, in particular for reasons of cost and suitability.